1. Field of the Invention
The present invention relates to a photomask, a method of manufacturing the photomask and a photomask processing device.
2. Discussion of the Background
Lithography has been a powerful technique for forming a highly-integrated circuit pattern on a semiconductor wafer. In this technique, a photomask having the original circuit pattern is illuminated with an exposure light, and the circuit pattern is transferred to the photoresist film coated on the wafer. The transferred image is desired to have a high resolution and contrast so as to form a fine, precise circuit pattern on the semiconductor wafer. However, the photomask can be deformed by heat or other causes, and the image obtained by such a deformed photomask may have a reduced contrast and an undesirable pattern. Therefore, various methods have been proposed and attempted to suppress such a photomask deformation.
For example, Japanese Unexamined Patent Publication 10-135121 discloses an exposure tool having a mask holder which is made of material having the same coefficient of thermal expansion as that of the photomask. The mask holder is also designed to hold the photomask by air suction through the suction holes provided on the surface of the mask holder.
Japanese Unexamined Patent Publication 11-195595 discloses an exposure tool in which the photomask is held vertically in the attempt to prevent from the deformation of the patterned surface of the photomask due to the influence of gravity.
Furthermore, Japanese Unexamined Patent Publication 11-194479 discloses a method of manufacturing a photomask in which the mask blank having flat surfaces is held so that the surface for the pattern formation faces upward and is curved in a convex form, and on this convex surface a circuit pattern is formed. The photomask having the pattern is then placed horizontally in the exposure tool with the patterned surface facing downward, and thus the surface of the substrate is deformed in a convex form due to the gravity effect. In order to compensate for this deformation of the surface, according to this method, the circuit pattern is provided on the convex-curved surface of the substrate.
The contents of the Japanese Unexamined Patent Publications 10-135121, 11-195595 and 11-194479 are incorporated herein by reference in their entirety.